1. Field of the Invention
The present invention relates to a method of preventing blockage of the nozzles and/or openings forming passages for combustion gases fed to a fluidized bed (e.g. a post-combustion bed in a hot water or steam boiler with two or more fluidizing beds). In such boilers. the bottom of the post-combustion bed can comprise two spaced-apart plates defining a plenum therebetween, the combustion gases passing vertically up through both plates through a nozzle and entraining air or gas from the plenum.
The invention also extends to an improved fluidized bed heating plant.
2. Description of the Prior Art
In known hot water or steam boilers with two or more fluidizing beds, the gases of combustion flow from the lowermost bed (the combustion bed) upwardly through the bed(s) positioned above it. In the bed immediately above the combustion bed (the post-combustion bed), the aim is to achieve final combustion of unburnt residual fuel, which is discharged from the underlying combustion bed. The purpose of the post-combustion bed is to achieve the best possible combustion efficiency.
To enable an efficient combustion of the combustion gases, air (secondary air) is supplied immediately before or in the vicinity of the post-combustion bed. The gas stream from the underlying combustion bed contains varying contents of solid particles in the form of unburnt fuel, ashes or bed material, for example. These entrained particles are intended to be either burnt in the post-combustion bed or to pass through this bed (and any subsequent beds) to be separated in a normal manner from the escaping flue gases in upstream cyclones and/or flue gas filters.
The bed bottom of the post-combustion bed and any subsequent beds is normally formed with openings (usually some form of nozzle arrangement), through which the combustion gases pass at high speed to bring about the necessary fluidization of the bed material (usually sand) in the post-combustion bed. The holes or the nozzles in a bed bottom are normally evenly distributed over the entire bottom plane of the bed.
When particle-contaminated combustion gases pass through the openings or nozzles there is a risk that the entrained solid particles will adhere to the openings or nozzles and cause at least partial blockage of the gas stream. Such problems may, of course, also arise in any fluidizing bed upstream of the combustion bed.